Atropisomerism of diflunisal unveiled by rotational spectroscopy and quantum chemical calculations

The most stable conformer of laser-ablated diflunisal has been isolated in a supersonic expansion and experimentally detected through high-resolution chirped-pulse rotational spectroscopy. State-of-the-art chemical calculations allowed to understand the nature of the strong stabilization of the detected conformer and its atropisomer among a total of sixteen theoretically predicted conformers and confirmed the presence of a resonance assisted hydrogen bond (RAHB) between the hydroxyl hydrogen atom and the carbonyl oxygen atom of the carboxylic acid group. The comparison of the experimental data from this work and the information found in the literature about the molecule in condensed phases corroborates the existence of these two atropisomers and is contextualized within the complexation arrangement of diflunisal with relevant proteins.